The general principles of coherent detection of modulated signals is well established. In the field of optical communication systems coherent receivers using laser local oscillators and balanced detectors have been described in "Laser Receivers" by Monte Ross, published by Weily, N.Y. In that book, at pages 112-113 there is described a receiver in which an input signal plus a local oscillator reference signal is fed to one of a pair of basically identical detectors, while the input signal minus the local oscillator reference signal is fed to the other detector of the pair. The arrangement requires, in addition to a beam splitter, two half silvered mirrors, a 180.degree. phase changer for the local oscillator signal, and a substrate circuit for the two detector outputs. The subtracted outputs from the two detectors forms the output signal.
Thus a conventional coherent optical receiver requires the summing of the received signal field E.sub.1 with a larger local oscillator field E.sub.2 and the resultant is detected at a photodetector PD (FIG. 1). As this has a square law sensitivity (E.sub.1 +E.sub.2) to field the output O/P contains a component 2E.sub.1, E.sub.2 proportional to the product of the two fields. Hence, the larger the local oscillator field E.sub.2 the greater the coherent gain. With an ideal local oscillator of great magnitude, the sensitivity is only limited by the quantum noise. However, in practice the local oscillator has some level of unwanted amplitude modulation (intensity noise) which appears in the component I.sub.2 and the larger the LO, the greater the AM noise which is coupled to the receiver output.
Previously it has been proposed that a balanced receiver would allow the AM noise to be balanced out. In British patent application No. 2121636A there is described a coherent optical receiver where the two outputs of the mixer are both detected and summed in antiphase in the receiving amplifier. This cancels large photocurrent variations due to the local oscillator.
The balanced input receiver is shown in FIG. 2 and is attractive as it minimizes the effect of stray capacitance in a two-detector receiver PD.sub.1, PD.sub.2 and it pulls out the large local oscillator (LO) dc photocurrent component which would otherwise cause problems of receiver dynamic range.
Alternatively, it may be achieved by electrically balancing the gains of two independent receivers as is described by ABBAS. G. et al in paper TUAS, pp 34, 35, Proc. OFC '84 and shown in FIG. 3.
The degree to which LO intensity fluctuations can be suppressed is determined by the accuracy of the balancing. A 10% error in (intensity) balance will produce a factor of ten improvement, 1% a factor of 100 etc. A perfect balance would be ideal but there are several impediments in achieving this. Couplers (power dividers) are usually sensitive to temperature and stress, pairs of photodetectors may have differing responsivity and fiber-to-detector losses may differ. Even if the balance can initially be set up to the optimum, small changes in ambient conditions will upset this, so there is little chance of obtaining sufficiently accurate and stable balancing to allow the fluctuations in a large local oscillator signal to be suppressed.